Field of the Invention
The present invention relates to a wireless relay device, a control device, a wireless communication system, and a joining method.
Priority is claimed on Japanese Patent Application No. 2015-168223, filed on Aug. 27, 2015, the content of which is incorporated herein by reference.
Description of Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
Conventionally, a distributed control system (DCS) has been constructed in plants or factories, and a highly automated operation has been realized. This distributed control system is a system in which a field device (a measurement device or an operating device) and a control device that performs control of the field device are connected via communication means. Some communication systems that are a basis of such a distributed control system perform wired communication. In recent years, a wireless communication system that performs wireless communication conforming to industrial wireless communication standards such as ISA100.11a or WirelessHART (registered trademark) has been realized.
In such a wireless communication system, a wireless network is centrally managed by one management device, mainly, from the viewpoint of prevention of compromises in security. For example, in a wireless communication system conforming to ISA100.11a described above, a joining process of causing field devices (wireless field devices) capable of wireless communication to join a wireless network, an encryption key distribution process, a process of allocating communication resources (time slots and communication channels), and other processes are performed by the management device called a system manager.
Japanese Unexamined Patent Application, First Publication No. 2014-86852 discloses an example of a wireless communication system conforming to, for example, ISA100.11a or WirelessHART (registered trademark) described above. Specifically, Japanese Unexamined Patent Application, First Publication No. 2014-86852 discloses an invention in which more wireless devices than those in a case of the related art can be caused to join the wireless network through withdrawal from a wireless network in a case in which a wireless device transmitting a request for joining the wireless network is caused to join the wireless network and wireless communication over the wireless network is completed.
The number of field devices provided in a plant or the like may generally increase or decrease according to a scale of the plant or the like, and may exceed 10000 in a large-scale plant or the like. Therefore, in a large-scale plant or the like, a large-scale wireless network (for example, a wireless network to which more than 10000 wireless field devices are connected) may be considered to be likely to be constructed in the future.
However, at present, since a wireless network is centrally managed by one management device, there is a problem in that it is difficult to construct a large-scale wireless network for the following reasons.
(1) Limit of Processing Capability of the Management Device
The number of wireless field devices that can connect to the wireless network is limited by the processing capability of the management device. In order to construct a large-scale wireless network, it is necessary to achieve improvement of the processing capability of the management device (for example, to perform a change to hardware in which a higher performance central processing unit (CPU) is mounted and more memories are mounted). However, since the improvement in processing capability of the management device is limited, it is difficult to construct a large-scale wireless network.
(2) Lack of Wireless Bandwidth Resources
In a wireless communication system, it is necessary to ensure a wireless bandwidth necessary for transmission and reception of data that is used for the joining process that is performed in order to cause a new wireless field device to join the wireless network, in addition to a wireless bandwidth necessary for a wireless field device that has already joined the wireless network to perform data transmission and reception. If a large number of field devices are caused to join the wireless network, the wireless bandwidth necessary for transmission and reception of data that is used for the joining process increases. Since in the wireless communication system, a wireless bandwidth that can be allocated is limited, the maximum number of wireless field devices that can connect to the wireless network is reduced by a wireless bandwidth allocated for transmission and reception of data used in the joining process. Thus, it is difficult to construct a large-scale wireless network due to lack of wireless bandwidth resources.
(3) Increase in Power Consumption
Data used for the above-described joining process is often relayed by a relay device (wireless router) arranged on a communication path between the wireless field device and the management device. If a large number of field devices are caused to join the wireless network, the data used for a joining process increases, and accordingly, power consumption of the relay device increases. If the amount of data relay is limited so that the power consumption of the relay device is equal to or less than a constant value, the number of wireless field devices that can join the wireless network is limited. Thus, if an increase in power consumption of the relay device is suppressed, it is difficult to construct a large-scale wireless network.
(4) Increase in Time Required for Joining Process
Since the data of the joining process is often relayed by the relay device (wireless router) arranged on the communication path between the wireless field device and the management device as described above, it takes a certain amount of time to complete the joining process. If more than a million wireless field devices are caused to join the wireless network, it takes a long time (for example, a few days) to complete the joining process of all of the wireless field devices. If the time required for the joining process is limited to a realistic time, the number of wireless field devices that can join the wireless network is limited. Thus, if the time required for the joining process is limited, it is difficult to construct a large-scale wireless network.